1. Field of the Invention
This invention relates to a liquid jet recording head which performs recording by jetting a liquid to form flying liquid droplets.
2. Description of the Prior Art
Ink jet recording methods (liquid jet recording methods) are recently attracting attention for such advantages that generation of noise during recording is negligible, that high speed recording is possible and also that recording can be done on so-called plain paper without need of a special treatment for fixing.
Such liquid jet recording methods are disclosed in, for example, Japanese Laid-open Patent Application No. 51837/1979, Deutsche Offenlegungsschrift (DOLS) No. 24843064 which have a specific feature different from other liquid jet recording methods in that the driving force for discharging liquid droplets is obtained by permitting heat energy to act on a liquid.
That is to say, according to the recording method disclosed in the above patent specifications, liquid which has received action of heat energy undergoes a change in state accompanied with an abrupt increase of volume, and through the acting force resulting form the change in state is discharged through the orifice at the tip end of the recording head portion to be formed into flying liquid droplets, which are attached onto a material to be recorded, thereby effecting recording thereon.
In particular, the liquid jet recording method disclosed in DOLS No. 2843064 is not only applicable very effectively for the so-called drop-on demand recording method, but also can easily be embodied into a recording head in which the recording head portion is made into a high density multi-orifice of full line type, thus being capable of giving images of high resolution and high quality at high speed.
The recording head portion of the device to be applied for the above-mentioned method has a liquid discharging portion having an orifice provided for discharging liquid and a liquid pathway, which is communicated with the orifice and has a heat acting zone at which heat energy acts on liquid for discharging liquid droplets, and an electrothermal transducer as a means for generating heat energy. And, the electrothermal transducer has a pair of electrodes and a heat generating resistance layer which is connected to these electrodes and has a region for heat generation (heat generating portion) between these electrodes.
A typical example exhibiting the structure of such a liquid jet recording head is shown in FIG. 1(A) and FIG. 1(B). FIG. 1(A) is the front view of a liquid jet recording head as viewed from the orifice side, and FIG. 1(B) is a partial sectional view of FIG. 1(A) when cut along the broken line X - Y.
The recording head 101 has a structure having orifices 105 and liquid discharging sections 106 formed by bonding a grooved plate 104 provided with a certain number of grooves of certain width and depth at a predetermined line density to a substrate 103 provided on its surface with an electrothermal transducer 102 so as to cover over the surface of the substrate 103. In the case of the recording head as shown in the drawing, it is shown as having a plural number of orifices 105. Of course, the present invention is not limited to such embodiments, but also a recording head with a single orifice is included in the category of the present invention.
The liquid discharging section 106 has an orifice 105 for discharging liquid at its terminal end and a heat acting zone 107, which is the place where heat energy generated from an electrothermal transducer 102 acts on liquid therein to generate bubbles and cause adrupt change in state through expansion and shrinkage of its volume. The heat acting zone 107 is positioned above the heat generating portion 108 of the electrothermal transducer 102 and has a heat acting face 109 to be contacted with the liquid at the heat generating portion 108 as its bottom surface.
The heat generating portion 108 is constituted of a lower layer 110, a heat generating resistance layer 111 provided on the lower layer 110 and an upper layer 112 provided on the heat generating resistance layer 111. The heat generating resistance layer 111 is provided on its surface with electrodes 113 and 114 for passage of current to the layer 111. The electrode 113 is common to the heat generating portions of the respective liquid discharging portions, while the electrode 114 is a selective electrode for selecting the heat generating portion of each liquid discharging portion for heat generation and is provided along the liquid pathway of the liquid discharging portion.
The upper layer 112 has the function of separating the heat generating resistance layer 111 from the liquid filling the liquid pathway of the liquid discharging portion for protection of the heat generating resistance layer 111 chemically or physically against the liquid employed at the heat generating portion 108, and also has the protective function for the heat generating resistance layer 111 to prevent short-circuit through the liquid between the electrodes 113 and 114.
The upper layer 112 also serves to be under charge of preventing electrical leak between adjacent electrodes. In particular, prevention of electrical leak between the respective selective electrodes or prevention of electric corrosion, which will occur by passage of current between the electrode under each liquid pathway and the liquid which may happen to be contacted for some reason, is important and for this purpose the upper layer 112 having such a protective function is provided at least on the electrode existing under the liquid pathway.
Further, the liquid pathway provided at each liquid discharging portion is connected upstream thereof to the common liquid chamber for storage of the liquid to be supplied to said liquid pathway, and the electrode connected to the electrothermal transducer provided at each liquid discharging portion is generally provided for convenience in designing so that it may pass beneath the aforesaid common liquid chamber on the side upstream of the heat acting zone.
Accordingly, it is generally practiced to provide the upper layer as described even at this portion in order to prevent contact between the electrode and the liquid.
Whereas, the above-mentioned upper layer 112 is required to have characteristics which are different depending on the place at which it is to be provided.
For example, at the heat generating portion 108, it is required to be excellent in (1) heat resistance, (2) liquid resistance, (3) liquid penetration preventing characteristics, (4) thermal conductivity, (5) antioxidant properties, (6) breaking resistance, while in regions other than the heat generating portion 108, it is required to be excellent sufficiently in liquid penetration preventing characteristics, liquid resistance and breaking resistance, although thermal conditions may be somewhat undemanding.
However, there is nowadays no material for constituting the upper layer which can satisfy all of the above characteristics (1) to (6) as desired, and under the present situation, some of the characteristics (1) to (6) are placed under alleviated requirements.
That is to say, the choice of material in the heat generating portion 108 is done with preference for characteristics (1), (4) and (5), while in other portions than the heat generating portion 108, for example, the electrode portion, the choice of material should be done with preference for characteristics (2), (3) and (6), thus forming the upper layers with the use of corresponding materials on the respective regional faces.
On the other hand, as different from these, in the case of a multi-orifice type liquid jet recording head, formation of respective layers and partial removal of the layers formed are conducted repeatedly on a substrate in the manufacturing step for the purpose of forming a number of minute electrothermal transducers at the same time on the substrate. At the stage when the upper layer is formed, the surface on which the upper layer is to be formed is formed in minute projection-recess shape with a slab wedge portion (or stepped portion), and therefore the step coverage characteristic of the upper layer at this stepped portion becomes important.
In short, if the step coverage characteristic at this stepped portion is bad, penetration of liquid will occur at that portion, whereby electric corrosion or breaking of electric insulation may be induced.
Also, when the upper layer is susceptible to occurrence of failures at a high probability in the manufacturing steps, liquid will penetrate through the failures to cause markedly reduced life of the electrothermal transducer.
For the reasons mentioned above, the upper layer is also required to have good coverage characteristics at the stepped portion, with a low probability of occurrence of pinholes in the layer formed, to the such probability is negligible or less.
However, in the prior art, no satisfactory liquid bubble jet recording head has been proposed, which can satisfy all of these requirements and is excellent in overall use durability.